1. Field of the Invention
The present invention relates to a multi-color image formation method and apparatus using an electrostatic latent image and in particular relates to a new multi-color image formation method and apparatus which transfers a toner image formed by repeated latent image formation and developing processes to a recording medium.
2. Discussion of the Related Art
One conventionally known multi-color image formation method of the above-mentioned type is disclosed by Japanese Patent Application Unexamined Publication No. Sho. 58-116553 (1983). The multi-color image formation method disclosed by the above-identified document uses, as FIG. 8 shows, a light-sensitive member 100 such as a light-sensitive drum and around it are installed an electrical charger 101, plural lasers 102 and 103 for exposing image portions to the light, first and second developing devices 104 and 105, respectively, storing toner of plural colors used for reverse development, a recharger 106 which recharges the light-sensitive member 100 and is located just behind the first developing device 104, a transfer device 107 transferring toner image of each color to a recording medium 112, a pretransfer lamp 108 which removes the surface potential of the light-sensitive member 100 to ensure smooth transfer operation by the transfer device 107, a discharge detachment means 109 for detaching the recording medium 112 from the light-sensitive member 100 after the transfer process, a cleaner 110 which removes residual toner from the surface of the light-sensitive member 100 and an erase lamp 111 which removes any charge remaining on the surface of the light-sensitive member 100; and in one cycle of the image formation, the surface of the light-sensitive member 100 is first changed, and a two-color toner image is formed on the light-sensitive member 100 by exposing an image portion and reverse developing twice and then the toner images of the colors are transferred together to the recording medium 112 in one cycle of the image formation.
However, as shown in FIG. 9, since the multi-color image formation method described above has a recharging stage after the first developing process, the recharger 106 is required; therefore the number of components increases and what is worse, the recharger 106 tends to be contaminated by toner particles detached from the first toner image or escaping from the first and second developing devices 104 and 105 because of the need for recharging after the first toner image and the recharger 106 is installed between the developing devices 104 and 105, which leads to deterioration of stability in the recharging process and results in a loss of quality of the multi-color image. In this figure, V.sub.B1 and V.sub.B2 indicate developing bias voltages of the developing devices 104 and 105, respectively.
On the other hand, in the method disclosed by Japanese Patent Application Unexamined Publication No. Sho. 56-52760 (1981), for example, after charging, first exposure of the image portion and first reverse development, recharging is not carried out, and the second exposure of the image portion and second reverse development immediately follow to provide a two-color image as shown in FIG. 10 where V.sub.B1 and V.sub.B2 indicate the developing bias voltages of the developing devices 104 and 105, respectively.
The above-described method does not require the recharger 106 as shown in FIG. 8, and therefore no deterioration of stability in the recharging process as seen in the method disclosed by the Japanese Patent Application Unexamined Publication No. Sho. 58-116553 occurs. However, in this method, because a latent image potential formed in the process of the first exposure of the image portion remains after the first development, the first image portion is again developed with second toner when the second development is carried out, thus causing a problem of color mixing.